This invention relates to spray coating systems and particularly to an improved nozzle assembly for spray coating guns. More specifically, this invention relates to an external air atomizing nozzle assembly for electrostatic spray guns such as that disclosed in Hastings et al U.S. Pat. No. 3,747,850 issued on July 24, 1973, and assigned to the assignee of this invention.
In conventional electrostatic spray systems, a fluid coating material such as paint, varnish, lacquer and the like is passed through the barrel of a spray gun, into a fluid tip which is threaded at its rear into a counterbore in the forward end of the barrel, and through and out of a small diameter nozzle at the forward end of the fluid tip. An air cap surrounds the forward end of the fluid tip and includes a central bore surrounding the nozzle so as to define an annular air passage around the fluid nozzle. Air issuing from this annular passage impacts with the stream of material issuing from the material orifice of the nozzle to at least coarsely atomize the material stream. There may be additional openings or ports in the air cap to further atomize or control the material stream as well as a pair of fan-shaping ports located in a pair of opposed horns of the air cap. A trigger operated valve controls the flow of air through the atomizing air passage, and a manually adjustable valve controls the amount of air issuing from the horn of the nozzle and thus the degree of "fan" formed by the atomized spray. Patents generally illustrating such systems are U.S. Pat. Nos. 1,655,254; 2,101,175; 2,138,300; 3,672,569; and 3,747,850.
In such systems, it is of utmost importance that the annular air passage defined by the wall of the central bore in the air cap and the outside diameter of the fluid tip nozzle be accurately concentric with the material orifice of the nozzle. If this concentricity deviates by as little as one or two one-thousandths of an inch, atomization of the material becomes non-uniform and the shape of the spray emitted from the gun becomes badly distorted. Because the fluid tip is supported at its rearward end or at an intermediate position still removed from the nozzle, it is extremely difficult to obtain the accurate alignment of the nozzle in the central bore. This is particularly true when the nozzle assembly is formed of a nonconductive material such as plastic since it is particularly difficult to manufacture plastic parts in the tolerances required to achieve concentricity.
The problem of controlling the atomization of the fluid material and the shape of the spray emitted from the gun increases as the flow rate of material through the gun decreases. In sum, very small variations in the annular air passage surrounding the fluid tip nozzle have been found to have very drastic effects on the shape of the spray pattern emitted from the gun.